The invention relates generally to hydrodynamic bearing assemblies of the type that provide support and rotation for high-speed spindle elements. Such hydrodynamic bearing assemblies can be utilized, for example, in computer disc drive recording systems.
Disc drive memory systems are used in computers for storage of digital information that can be recorded on concentric tracks of a magnetic disc medium. One or more discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm which moves radially over the surface of the disc. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
During operation, the discs are rotated at high speeds within an enclosed housing using an electric motor located inside a hub or below the discs. One type of motor in known as an in-hub or in-spindle motor. Such motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is located near the top of the spindle and the other near the bottom. The bearings permit rotational movement between the shaft and the hub while maintaining proper alignment of the spindle to the shaft. The bearings can be lubricated with grease or oil.
The conventional bearing system described above is prone to several problems, including vibrations generated by the balls rolling on the associated raceways. The strict requirements of shock resistance for hard disc drives in portable computer equipment also makes the use of such conventional systems less desirable. Another problem relates to the fact that mechanical bearings are not always scalable to smaller dimensions. That is a significant drawback because the trend in the disc drive industry has been to continually reduce the physical dimensions of the disc drive unit.
As an alternative to the conventional ball bearing systems, hydrodynamic bearing systems have been developed. In a hydrodynamic bearing system, a lubricating fluid, such as a gas or liquid, serves as the bearing surface between a stationary base or housing and the rotating spindle or rotating hub. The size of the gap between the rotating hub and the stationary portion of the motor must be tightly controlled to obtain good dynamic performance.
Unfortunately, the control required for the dimensions of the gap makes machining those sections costly. Furthermore, variations in the manufacturing process that result from machining metal sections of the disc drive system make it difficult to obtain a gap with specified dimensions in a repeatable fashion.
In general, a spindle motor assembly includes a polymeric motor shaft, a polymeric hub to support a rotating disc, and a stator disposed in an internal cavity defined by the hub. An outer surface of the motor shaft and an opposing surface of the hub form a hydrodynamic journal bearing.
One or more of the following features may be included in various implementations. A resilient snap-in retainer can be molded to the hub to hold the disc in place. Alternatively, a fastener molded to the hub can hold the disc in place. A resilient snap-in retainer can be molded to the motor shaft to hold the hub in place.
The stator can include coils and one or more magnets can be connected to the hub such that during operation of the disc drive, the magnets interact with the coils to cause rotational movement of the hub about the motor shaft. The magnets can be attached to a back-iron that is attached to the hub. Additionally, a flux conducting ring can be disposed at an outer perimeter of a horizontal extension of the motor shaft. The flux conducting ring can provide a thrust bearing to facilitate operation of the spindle motor assembly.
Pressure generating features can be formed in the polymeric motor shaft. During operation, the journal bearing and pressure generating features create a pressure gradient in a gap between the surface of the motor shaft and the opposing surface of the hub. The pressure generating features can include, for example, spiral grooves formed in the motor shaft or Rayleigh steps formed in the motor shaft.
A solid lubricant can be disposed on the hub on a surface where the hub contacts the motor shaft during operation of the spindle motor assembly. Similarly, a solid lubricant can be disposed on a surface of the motor shaft that contacts the hub during operation of the spindle motor assembly.
The spindle motor assembly can form, for example, part of a computer disc drive.
According to another aspect, a method of assembling a spindle motor assembly for a computer disc drive includes positioning a hub over a motor shaft, pressing the hub downward so that an extension on the hub contacts a snap-in retainer molded to the motor shaft, and allowing the snap-in retainer to spring back to hold the hub in place.
In some implementations, the hub defines an internal cavity for a stator when the hub is held in place by the snap-in retainer. A disc can be held in place with a snap-in retainer molded to the hub.
Some implementations include one or more of the following advantages. Forming the hub and/or motor shaft with a polymeric material can facilitate the achievement of tight control of dimensions of those components during fabrication to obtain improved dynamic performance. Use of polymeric materials also can result in more repeatable manufacturing techniques. Known techniques, such a mold injection, can be incorporated into the manufacturing process to make the hub and motor shaft, and other components can easily be molded or otherwise connected as part of the spindle motor assembly. Similarly, a pattern of grooves, Rayleigh steps or other features can be formed on the motor shaft during the mold injection process to provide the appropriate pressure gradients for stabilizing the spindle motor during operation of the disc drive.
Use of polymeric materials for the hub and/or motor shaft also can facilitate the assembly process of the disc drive using snap-in features that easily can be formed by injection molding or other techniques. Such snap-in features can be used, for example, to hold the hub in place with respect to the motor shaft and to hold a disc in place.
Use of polymeric materials can, in some cases, provide a significant reduction in manufacturing and assembly costs because the various components can be made smaller.
Providing solid lubricants on selected areas of the of the surface of the hub and/or motor shaft can improve the tribology and reduce the amount of liquid lubricant that might otherwise be required as a result of absorption of the liquid lubricant by the polymeric hub or motor shaft.
Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claims.